Mass spectrometry glycophenotype characterization of ALG2-CDG in Argentinean patients with a new genetic variant in homozygosis

Abstract

Human ALG2 encodes an α 1,3mannosyltransferase that catalyzes the first steps in the synthesis of N-glycans in the endoplasmic reticulum. Variants in ALG2cause a congenital disorder of glycosylation (CDG) known as ALG2-CDG. Up to date, nine ALG2-CDG patients have been reported worldwide. ALG2-CDG is a rare autosomal recessive inherited disorder characterized by neurological involvement, convulsive syndrome of unknown origin, axial hypotonia, and mental and motor regression. In this study, we used MALDI-TOF MS to define both total serum protein and transferrin (Tf) N-glycan phenotypes in three ALG2-CDG patients carrying a c.752G > T, p.Arg251Leu ALG2 missense variant in homozygous state, as determined by exome sequencing. Comparing it to control samples, we have observed Tf under-occupancy of glycosylation site(s) typical of a defective N-glycan assembly and the occurrence of oligomannose and hybrid type N-glycans. Moreover, we have observed a slight oligomannose accumulation in total serum glyco-profiles. The increased heterogeneity of serum N-glycome in the studied patients suggests a marginal disarrangement of the glycan processing in ALG2-CDG. Previous studies reported on slightly increased concentrations of abnormal serum N-glycans in CDG-I due to defects in the mannosylation steps of dolichol-linked oligosaccharide biosynthesis. This preliminary work aims at considering serum N-glycan accumulation of high mannosylated glycoforms, such as oligomannose and hybrid type N-glycans, as potential diagnostic signals for ALG2-CDG patients.

Epidemiology of congenital disorders of glycosylation (CDG)—overview and perspectives

“…The South American country with the highest number of reported CDG patients is Argentina [48,97,98]. There may be underdiagnosis in other South American countries, but the high influx of European immigrants (and thus, genetic import) in the last century could also have contributed to this observation in Argentina.…”

Glycan quality control in and out of the endoplasmic reticulum of mammalian cells

“…This hypoglycosylation phenotype occurs, because (a) DLO intermediates are not efficiently used by OST [23] and (b) the supply of the fully assembled DLO is substantially reduced, highlighting the critical role of normal DLO biosynthesis in achieving maximal N- glycosylation. Accumulating evidence indicates that incompletely assembled DLOs are not only rarely transferred to proteins in cells of CDG-I patients [88], but they are also rapidly degraded by the action of a putative pyrophosphatase (Fig. 4A) [17,18], suggesting that glycan QCS exists to eliminate defective DLOs.…”

Dystroglycanopathies: Genetic Bases of Muscular Dystrophies Due to Alteration in the O-Glycosylation of α-Dystroglycan

“…The glycosylation is a complex biological process involving many pathways. CDG are sub-grouped into defects of protein N-glycosylation, protein O-glycosylation, lipid glycosylation, GDP-anchor glycosylation and multiple glycosylation defects [81,86]. O-glycosylation is a common covalent modification of serine and threonine residues of mammalian glycoproteins [53].…”

“…Elucidation of the molecular pathological mechanisms of CDG associated with DG glycosylation abnormalities will be an important issue in understanding the mechanisms of dystroglycanopathies. Characterizing a glycoproteome profile of patients prior to and on treatment will help to better understand the changes of a plethora of glycoproteins and related clinical observations in dystroglycanopathies [81][82]86,97].…”